Re: Dumb question...go ahead and crush me..
Saw your boat in another post - you made a killer buy. Good job!
Yep, it's your drain hole. More importantly, it's the hole in which to put something to stop water from comin' in

I've not heard of a threaded one before, though. If you've got the plug for it, check it against what you see available at a Wally World or your local marine store (just to make sure you can get replacements). You'll want to have a spare with you at all times. I've had mine (not threaded) pop out once before.
You said it's your first I/O, but it didn't sound like it's your first boat. If you're not familiar with running a boat this size, it might well be helpful to get someone to help you get oriented to it. I just made the jump from outboards to I/O's last year and have learned a bunch of lessons. Ultimately, I like this new (to me) boat more - but there have been plenty of lessons to learn.
Make sure you know what you're doing before you get out on the water, it can be costly and dangerous if you don't. But it's not rocket science either - you're gonna' have a blast.